The present invention relates to a method and apparatus for analysis of submicron-sized particles, such as soot, over a wide range of particle concentrations with high temporal and spatial resolution in particular, it relates to improvements in the Laser-Induced Incandescence technique (LII for short) for improved measurement accuracy by the use of a laser beam of low fluence and/or a good laser energy profile.
The presence of particulate matter, such as soot particles, in the environment has brought about an increased interest in the development of methods and devices for the determination of particulate concentration and its average sizes. Soot in particular has been the subject of study for measurement. However, all small particles pose an important area of interest and concern, particularly for environmental and health reasons. The emission of soot from engines, power generation facilities, incinerators, or furnaces, for example, represents a loss of useful energy and further is a serious environmental pollutant and a health risk. However, the presence of soot in flames can also have positive effects. For example, the energy transfer from a combustion process is largely facilitated by the radiative heat transfer from soot. Thus, to understand soot formation and develop control strategies for soot emission or formation, measurements of soot concentrations are necessary. Other applications include characterization of metal nanoparticles and ceramic nanoparticles. The characterization can be used for monitoring, regulatory compliance, process control production of value-added nanoparticles, and many other applications. LII is a good diagnostic tool for measurements of particulate as the LII signal is proportional to particle volume fraction and is also related to particle sizes.
Current techniques for measuring diesel particulate concentration include the Bosch Smoke Number and direct mass sampling. In the Bosch Smoke Number method particles are collected on filter paper from a portion of the exhaust stream and the light reflection from the collected sample is measured. This is compared against a calibration chart to determine the mass flow. Since sufficient sample material must be collected over time, this method requires a long period for sample collection and has a poor time and spatial resolution. Thus this method cannot provide diagnostic information about the formation of particles in the combustion cycle. The direct mass sampling method is the official regulatory method of the EPA and measures the mass of soot from a difference of the mass of the soot on a filter and the mass of the filter alone. This method, however, has a limited accuracy, particularly for low emission vehicles. Both methods suffer a loss in accuracy when the source produces lower emissions, and require significantly longer testing for low emission combustors.
The measurement of soot particle concentrations has been greatly improved by the development of LII, which can provide concentration information with high temporal and spatial resolution. Previous techniques could not detect small concentrations and could not provide accurate time resolved information regarding soot formation.
LII exposes a volume of gas containing refractory particles, which are particles capable of absorbing laser light energy with an evaporation temperature sufficiently high to produce measurable incandescence, to a pulsed, focused, high-intensity laser light. The particles absorb laser energy, heating to temperatures far above the surrounding gas. At these elevated temperatures (in a range of 4000-4500 K in the case of soot) the particles incandesce strongly throughout the visible and near infrared region of the spectrum. In the past, the regime in which evaporation was the predominant heat loss mechanism limited the maximum particle temperature. For example, any further increase in laser light energy resulted in an increase in the evaporation rate rather than an increase in particle temperature. In accordance with Planck""s radiation law, any material gives off energy in the form of radiation having a spectrum and magnitude influenced by its temperature. The higher the temperature is, the greater the intensity is and the shorter the peak wavelength is. Thus the radiative emission at these elevated temperatures increases in intensity and shifts to blue (shorter) wavelengths, compared with that of the surrounding medium. Thus the LII signal is readily isolated from any natural flame emission. Because of the rapid time scale and good spatial resolution, as well as its large dynamic range, LII is well suited as an optical diagnostic to measure soot volume fraction and the particle sizes in turbulent and time varying combustion devices. What was not appreciated heretofore was that optimum results could be achieved by controlling the maximum temperature to be less than a temperature such that evaporation never becomes the predominant heat loss mechanism for a majority of particles within a sample. Therefore, in accordance with this invention, it has been found that optimum results can be obtained by ensuring that no more than 5% of the total solid volume of the particles to be analyzed should be evaporated. Stated differently, preferably 95% of the total solid volume v; of the particles should not evaporate. In a most preferred embodiment less than 2% and preferably 1% or less of the total solid volume of the particles will be evaporated.
Hence, it is an object of this invention to provide a system wherein at least a majority of particles in a sample are heated such that they incandesce and do not significantly evaporate losing a substantial quantity of their solid volume, thereby cooling by way of conduction to a surrounding gas, or medium, rather than through significant evaporation as occurred in the past.
There is an important distinction that is made between xe2x80x9cthe inventionxe2x80x9d and the prior art described heretofore. By way of example, the prior art system above, heated a majority of soot particles in a sample, to elevated temperatures between 4000-4500K. At these elevated temperatures two cooling mechanisms were at play; evaporation, and conduction. It was believed at the time, that an advantage of heating particles to these high temperatures was that they incandesced strongly; another advantage was that the LII signal generated was relatively independent of laser fluence, although for unknown reasons; and it was believed that this was an optimum condition.
In patent application WO 97/30335 in the names of Alfred Leipertz et al., published Aug. 21, 1997, a laser-induced incandescence technique is described for determining a primary particle size. The technique taught by Leipertz includes the measurement of the incandescence at two discrete points in time after the laser light pulse, from which a ratio is generated to calculate the particle size according to a mathematical model. However, this technique has been shown to be prone to inaccuracies. Leipertz et al. sample the two measurements at a point of decay where they assume a linear change. This, however, is unlikely to happen until significant cooling has occurred and most of the signal has passed. Thus the signals measured by Leipertz et al are very weak and are highly influenced by noise. Laser fluence (spatial energy density) over the volume measured is also critical to the subsequent temperature decay. It is critical for accuracy to know the energy density profile over the volume. This factor is assumed without verification by the technique of Leipertz et al. Further error is introduced by the detection method, which uses spectrally broadband detectors to measure the signal. The Leipertz et al technique, as a result of these introduced errors, does not provide a good measurement of particle size.
Attempts to characterize particle size are also disclosed in a paper xe2x80x9cSoot diagnostics using laser-induced incandescence in flames and exhaust flowsxe2x80x9d by R. T. Wainner and J. M. Seitzman, published in 1999, by the American Institute of Aeronautics and Astronautics. This article reviews a method to determine particle size by measuring the peak temperature attained (pyrometry) by LII. However, the study found that the temperature of different-sized particles can be identical and thus temperature measurement at the peak is not sufficient to determine particle size.
The present inventors"" earlier U.S. Pat. No. 6,154,277 Nov. 28, 2000 and U.S. Pat. No. 6,181,419 Jan. 30, 2001 describe improvements in the LII technique.
U.S. Pat. No. 6,154,277 is directed to absolute intensity measurements in laser-induced incandescence. The invention relates to a method and an apparatus for the determination of particle volume fractions with LII using absolute light intensity measurements. This requires knowledge of the particle temperature either from a numerical model of particulate heating or experimental observation of the particulate temperature. The sensitivity of the detection system is determined by calibrating an extended source of known radiance and then this sensitivity is used to generate absolute LII signals. Further, by using a known particle temperature a particle volume fraction is calculated. This avoids the need for a calibration in a source of particles with a known particle volume fraction or particle concentration. This results in a calibration independent method and apparatus for measuring particle volume fraction or particle concentrations. A modeling process involves a solution of the differential equations describing the heat/energy transfer of the particle and surrounding gas, including parameters to describe vaporization, heat transfer to the medium, particle heating etc. The solution gives the theoretical particle temperature as a function of time.
U.S. Pat. No. 6,181,419 is directed to determining a primary particle size in laser-induced incandescence. The invention relates to a method and apparatus for applying LII to determine a primary particle size of submicron-sized particles. In addition to volume fraction information, particle size can be determined using LII due to the fact that transient cooling is dependent on the diameter of the particle. The ratio of a prompt and a time-integrated measurement from the same laser pulse has been found to be a function of the particle size. A modeling process is the same as that described in the above referenced U.S. Pat. No. 6,154,277. Thus the technique is able to provide more accurate measurements of particle size and particle volume fraction than previous LII techniques, particularly where time averaging is not possible and size measurements must be obtained from a single laser pulse. Calibration is needed to obtain a quantified volume fraction measurement.
In both of the above referenced U.S. Patents, it is stated in essence: Creating a well defined known laser light fluence (laser light energy per unit area, e.g., Joules/cm2) with minimal variation over the measurement volume is important since the incandescent signal is highly dependent on the laser light energy intensity profile. In the model, energy values for particles other than at the peak light intensity is calculated using a uniform distribution of particles about the optic axis aligned with the Gaussian light intensity profile. The particles not located at the peak receive proportionally less light energy and produce a different incandescence signal, as determined in the calibration, which is added cumulatively to determine a total incandescence signal for a given time step. While a Gaussian light intensity distribution of the fluence or light energy is often used, a xe2x80x9ctop-hatxe2x80x9d or square light intensity profile of the laser fluence having a constant light intensity throughout the laser light sheet would be beneficial. In principle any distribution of intensity can be used provided that its distribution through the measurement volume is measured. However, a more uniform light intensity profile ensures that the particulate temperatures are more uniform throughout the measurement volume. This increases the ease and accuracy of the numerical modeling and ensures that the average particulate temperature obtained from multi-wavelengths particulate measurements is more representative of the particle temperature in the measurement volume.
The said patents describe in detail an arrangement that creates a laser light sheet at the volume of the measurement location having a Gaussian fit profile of energy distribution (or fluence) in substantially one plane only. The profile of laser beam light fluence is flat in two orthogonal planes, the third plane being a Gaussian. Such profile is therefore not a true xe2x80x9ctop-hatxe2x80x9d profile and the numerical modeling is required to compensate the effect of varying fluence. With the true xe2x80x9ctop-hatxe2x80x9d profile (a constant low fluence excitation), the results of the numerical modeling are not required to determine the particle volume fraction.
Furthermore, prior work on LII has focused on moderate to high fluence to heat soot particles up to about 4500 K or above where LII signals reach a peak and the soot particles reach evaporation temperatures. This operating point is attractive in that LII signals are relatively insensitive to laser energy (or more precisely laser fluence). At those temperatures, however, the particles are being at least partially evaporated. At temperatures of 4000 K and above, the heat loss of the particles is dominated by evaporation, whereas conduction to the surrounding gas is dominant at lower temperatures. In this specification, therefore, the evaporation temperatures of a particle is defined as the temperatures at which evaporation replaces conduction as a dominant heat loss mechanism of the particle. For soot, therefore, the evaporation temperatures are in the range from 4000 K to about 4500 K, but particles composed of other materials may have different evaporation temperatures. With high evaporation, the particulate is surrounded by a cloud of superheated vapor, which affects the conduction-cooling rate of the particles and therefore affects the temperature decay rate. This, in turn, adversely affects the measurement of primary particle size because the temperature decay rate is proportional to the specific surface area (surface area per unit volume), which is used to determine the particle size. Furthermore, significant evaporation leads to a change in the total particle volume fraction measured and to the final primary particle size. In addition currently available models are not able to accurately predict the cooling behavior in this evaporation regime.
In accordance with this invention, it has been determined that LII signals do not have to be at or near the peak intensity to be measured and thus a laser light of low fluence may be used for LII measurements. With a high fluence laser light, the LII signals and particle temperatures are rapidly changing during the laser pulse due to rapid heating and evaporation of particles. Without evaporation, however, particles go through a relatively smooth conduction phase and produce an initially slower time constant temperature decay due to conduction cooling to the surrounding gas. With no interference from particle evaporation, the time dependent temperature decay reflects more accurately the particle size.
Furthermore, measurements can be made throughout the analyzing period until LII signals drop to the noise level of detectors. By avoiding significant particle evaporation, the concentration and primary particle size do not change during the measurement period, enhancing the reliability, ease, precision, and accuracy of the LII technique. To measure the temperature of particles, the two-color pyrometry technique is used in that the ratio of LII signals measured at two or more wavelengths indicate the temperature of particles. The temperature is measured at many points in time to generate the time dependent temperature decay characteristics.
In one aspect, the present invention relates to an improvement in LII and it uses a laser beam of low fluence at the measurement location to avoid heating the particle to a temperature where evaporation is the dominant heat loss mechanism. The temperature of the particles is measured and time dependent decay of the particle temperature is used to analyze the characteristics of the particles.
In a further aspect, the invention uses the two-color pyrometry technique to measure soot particle temperature as a function of time. In other words, it measures LII signals at two or more wavelengths and derives the temperature of soot particles at many points in time. It analyzes a time dependent decaying of the derived temperature of the particles. The decaying of the temperature is indicative of the characteristics of the particles, particularly the size.
In a yet further specific aspect, as LII signals are sensitive to laser energy distribution (fluence), the present invention employs a relay imaging optical arrangement that produces a very uniform fluence profile (also called xe2x80x9ctop-hatxe2x80x9d profile or distribution) throughout the measurement volume. This results in further improvements in accuracy of the LII technique of the present invention as the effect of varying fluence needs not to be compensated by means of the numerical modeling.
In accordance with another aspect of the invention, a method is disclosed for analyzing submicron-sized particles in a defined volume of gas. The method includes steps of heating one or more particles with a pulsed laser light beam to a temperature high enough for the particles to incandesce but less than an evaporation level of the particles and measuring incandescence from the particles at two or more wavelengths at a plurality of time intervals. The method further includes steps of calculating temperatures of the particles from the measured incandescence at a plurality of time intervals, and analyzing the calculated temperatures to obtain characteristics of the particles.
In accordance with a broad aspect of the invention, there is provided a method of analyzing a plurality of submicron sized particles having a total solid volume vi within a volume of gas, comprising steps of: (a) heating the plurality of the particles to be analyzed with a laser light beam to a temperature such that a majority the submicron particles measurably incandesce, while ensuring that the temperature is sufficiently low such that no more than 5% of the solid volume v; is evaporated; (b) measuring incandescence from the particles at one or more wavelengths; and (c) determining a characteristic of the particles in dependence upon the measured incandescence in step (b).
In accordance with yet another specific aspect, based on the experimentally derived temperature of particles using a low fluence laser light of non uniform profile, the invention uses the numerical modeling which involves a solution of a differential equations describing the heat energy transfer (heating and cooling) of particles and surrounding gas, to calculate the absolute LII intensities and then generates the soot volume fraction and particle size.
In accordance with another aspect, the method of the invention includes steps of generating a pulsed laser light beam of energy high enough to heat the particles to incandescence, passing the laser beam through an aperture and forming a relay image of the aperture at a measurement location located within the defined volume of gas. The method further includes steps of measuring incandescence from the particles at the measurement location at two or more wavelengths at a plurality of time intervals, calculating temperatures of the particles from the measured incandescence; and analyzing the calculated temperatures to determine characteristics of the particles.
In accordance with a yet further aspect, the invention is directed to an apparatus for analyzing submicron sized particles in a defined volume of gas by using laser-induced incandescence. The apparatus includes a laser for generating a pulsed laser light beam of a predetermined fluence and an optical arrangement including an aperture in an optical path of the pulsed laser light beam for limiting the transmitted pulse to an area of substantially constant fluence; imaging optics for forming a relay image of the aperture at a measurement location located within the defined volume of gas so that one or more particles in the defined volume of gas incandesce. The apparatus further includes at least one photodetector for measuring incandescence from the particles at two or more wavelengths at a plurality of time intervals, a signal processing unit for calculating temperatures of the particles at a plurality of time intervals and a signal analyzer for analyzing a time dependent decaying of the calculated temperatures to obtain characteristics of the particles.
In accordance with a further aspect, an apparatus of the invention includes a laser for generating a pulsed laser light beam of a predetermined fluence, and an optical arrangement for directing the pulsed laser light beam to heat the particles to a temperature high enough for the particles to incandesce but less than an evaporation level of the particles. The apparatus further includes at least one photodetector for measuring incandescence from the particles at two or more wavelengths at a plurality of intervals, a signal processing unit for calculating temperatures of the particles at a plurality of intervals and a signal analyzer for analyzing a time dependent decaying of the calculated temperatures to obtain characteristics of the particles.
In accordance with a specific aspect of the invention, a sample of particles, for example soot, can be analyzed by ensuring that a substantial majority of the sample of particles reach a temperature that will allow them to measurably incandesce and wherein the temperature of at least 80% and preferably 95% or more of the particles does not exceed 3900 K during the heating and detecting.
It is a significant advantage that the technique can provide more accurate measurements with high temporal and spatial resolution from a single laser light pulse, even for low particle concentrations. This is in part because of the use of more uniform energy distribution or xe2x80x9ctop-hatxe2x80x9d distribution of the laser light, and further to the reduction in errors due to evaporation effects.
A further advantage is that the apparatus in accordance with the present invention adapts the LII technique for in situ application, particularly with the convenience of absolute intensity measurements without the need for an additional calibration setup.
In summary improved accuracy on the volume particle fraction is obtained using the method in accordance with this invention. Simply stated, if a substantial amount of volume of the particles is evaporating during the process of analyzing a characteristic of the particles, optimum analysis is not achieved. In fact, the accuracy of the analysis is inversely related to the amount of solid particle volume loss.
Furthermore, improved accuracy for the measurement of the primary particle diameter is dependent upon the cooling rate of the particles; since the cooling rate of the particles is affected when they cool into an atmosphere of cooling plasma, i.e. evaporating particle gases, this limitation is substantially obviated by ensuring that significant evaporation and hence solid volume loss does not occur.
Additional advantages will be understood to persons of skill in the art from the detailed description of preferred embodiments, by way of example only, with reference to the following figures.